fix #18964 Small string case with else statement first in AST evaluat… (#20862)

fix #18964 Small string case with else statement first in AST evaluates wrongly

tests/casestmt/t18964.nim

@@ -0,0 +1,12 @@
+discard """
+errormsg: "invalid order of case branches"
+"""
+
+import macros
+
+macro genCase(val: string): untyped =
+  result = nnkCaseStmt.newTree(val,
+    nnkElse.newTree(quote do: echo "else"),
+    nnkOfBranch.newTree(newLit("miauz"), quote do: echo "first branch"))
+
+genCase("miauz")